1. Field of the Invention
The present invention relates to current sources and, more specifically, to cascode current sources operable at low and variable voltages.
2. Description of the Related Art
Current sources are widely used in analog circuits. As DC biasing elements, current sources are used extensively to establish the DC bias levels within a circuit while providing low sensitivity to power supply and temperature variations of the overall circuit. Current sources are also widely used as load devices in amplifier stages. The high incremental impedance of the current mirror provides a high voltage gain of amplifier stages at low power supply voltages.
FIG. 1 illustrates a current source 20 which includes three identical PMOS transistors 22, 24, and 26 that provide currents in respective branches 21, 23 and 25. Output node N40 of branch 21 is connected to the gate and the drain terminals of NMOS transistor 10. The source terminal of NMOS transistor 10 is connected to ground. Output node N42 of branch 13 is connected to the emitter terminal of PNP transistor 11. The collector and the base terminals of transistor 11 are connected to ground. Output node N44 of branch 25 is connected to one end of resistor 12. A second end of resistor 12 is connected to ground.
Because the gate and the source terminals of transistors 22, 24 and 26 are connected to respective nodes N46 and N45, transistors 22, 24 and 26 have substantially identical gate-to-source voltages. Consequently, the major source of mismatch between the magnitudes of currents I27, I28, or I29 is caused by differences between the values of the voltage signals at output nodes N40, N42, and N44. Differences between currents at output nodes N40, N42 and N44 is also caused in part by noise or mismatches in the sizes of PMOS transistors 22, 24, or 26. The differences in current also cause voltage differences at nodes N40, N42, and N44.
To lessen the dependence of the magnitudes of currents I27, I28, and I29 on the values of voltages at respective output nodes N40, N42, and N44 and thus to achieve a good matching between the magnitudes of currents I27-I29, it is desirable that the small signal output impedance of output nodes N40, N42, and N44 be high. A conventional technique for increasing the output impedance of a current source is to use a cascode configuration.
FIG. 2 illustrates a three-branch cascode current source 60 that is similar to current source 20 of FIG. 1, except that current source 60 uses cascode transistors 13, 14, and 15 in branches 21, 23, and 25, respectively. An input biasing circuit 40 establishes a voltage at node N50 less than the voltage at node N45. Transistors 13, 14, and 15 increase the impedances at output nodes N40, N42, and N44, respectively. Thus, current source 60 provides a much improved matching among the magnitudes of currents I27, I28, and I29 compared to current source 20, shown in FIG. 1.
The cascode configuration of current source 60 achieves a good current matching when the voltage across voltage supply V1 and ground, exceeds a minimum threshold. However, the trend is that the available voltage at V1 has decreased due system designs. When the voltage at V1 falls below a minimum threshold limit, e.g. 2.0 volts, and the voltage between nodes N50 and N45 is less than V1, e.g. 1.5 volts, a voltage across the drain-to-source terminals of cascode transistors 13, 14, and 15 becomes negligible, thereby rendering current mirror 60 inoperable at low supply voltages. Thus, for acceptable operation of current source 60, more supply voltage is required than is available.
Therefore, what is needed is a current source with a high output impedance that is also capable of operating from low supply voltages.